Methane and nitrous oxide concentration and emission flux of Yangtze Delta plain river net

Abstract

Methane (CH4) and nitrous oxide (N2O) saturation concentration and gas-water interface emission flux in surface water of the Yangtze Delta plain river net were investigated in summer at representative sites including the upper reaches of the Huangpu River and the rivers in the Chongming Island. The results show that the CH4 concentration in river water ranged from 0.30±0.03 to 6.66±0.14 mol·L−1, and N2O concentration ranged from 13.8±2.33 to 435±116 nmol L−1. River surface water had a very high saturation level of CH4 (from 468±49.0% to 11560±235%) and that of N2O (from 175±29.5% to 4914±1304%). Dissolved oxygen (DO) was the primary factor controlling the CH4 concentration in water. N2O concentration had significant negative correlation with salinity and a significant positive correlation with nitrate (NO3− ), nitrite (NO2− ), chemical oxygen demand (CODcr) concentration and pH of river water. CH4 and N2O of river water were brought about mainly by methanogenesis and denitrification in river bottom sediment that diffused through sediment-water interface into the water body and then into atmosphere through the gas-water interface. The emission flux of CH4 and N2O at river gas-water interface reached 778±59.8 and 236±63.6 mol·m−2·h−1, respectively in summer. The river net was a potential source of atmospheric CH4 and N2O because of eutrophication of the water body.